1. Field of the Invention
The present invention relates to a milling apparatus and a milling method wherein an object material is milled by colliding the object material against each other within a mill. The present invention also relates to a sintered body manufactured from a powder obtained by the above milling method.
2. Description of the Related Art
A conventional milling apparatus of the above kind comprises a mill for milling of the object material, a raw material feeder for supply of the object material to the mill, and a collecting tank for collection of a powder obtained by the mill. The mill includes a nozzle for injection of a high-velocity blast of an inert gas, and a classifying rotor for classified collection of the milled powder.
In the mill, the object material fed by the raw material feeder are collided against each other into small grains by the high-velocity blast of the inert gas injected from the nozzle. The milled powder is taken out by the classifying rotor and then collected in the collecting tank.
During the above-described operation, in order to obtain a uniform grain size distribution in the powder collected in the collecting tank, a constant milling condition, i.e. a constant condition for the collision in the mill, must be maintained. Then, in order to maintain the constant milling condition, the amount of the object material present in the mill must be maintained at a constant level.
Conventionally, in order to maintain the amount of the object material present in the mill at a constant level, the amount of the object material fed by the raw material feeder is varied by detecting weight change of the collecting tank.
Another method is to detect a load current to an electric motor which drives the classifying rotor. This detection method uses a principle that the load to the electric motor is increased by an increased amount of the object material colliding against the classifying rotor when there is an increase in the amount of the object material in circulation within the mill. Thus, the amount of the object material fed by the raw material feeder is reduced when the load to the electric motor becomes not smaller than a predetermined value.
Still another method of control is to provide a sensor for detecting a level of the raw material in the mill in order to determine if the amount of the object material in the mill has reached a predetermined level. In this method the feed of the object material from the raw material feeder must be continued unless the predetermined level is detected.
However, according to these conventional methods, it is difficult to maintain the amount of the object material remaining in the mill at a constant level.
Specifically, according to the method of detecting the weight of the collecting tank, the amount of the object material actually remaining in the mill can only be detected with a certain time lag. Therefore, since a decrease in the amount of the object material from a predetermined level cannot be detected soon, there is always a delay in supply timing of the object material. Furthermore, the object material is always overfed even if there is already enough amount of the object material supplied.
On the other hand, according to the method of detecting the load current to the electric motor which drives the classifying rotor, although there is less problem of the time lag than in the method of detecting the weight of the collecting tank, it is impossible to make detection that allows precise measurement of a change in the amount of the object material.
According to the method of providing the raw-material level sensor in the mill, it is possible to determine if the amount of the object material has reached the predetermined level, without the time lag. However, it is impossible to determine how much of the object material should be added when the level has not yet reached to the predetermined level. Therefore, there is a possibility that the amount of feed of the object material is being increased gradually in an attempt to achieve the predetermined level, yet actually the mill is discharging a greater amount, further reducing the amount of the object material present in the mill from the predetermined amount. If a large amount of the object material is fed at one time in an attempt to eliminate such a possibility as above, the amount in the mill can easily go far beyond the predetermined level. Further, the level sensor can not detect the level of the object material accurately, since the object material is blown up in the mill.
It is therefore a primary object of the present invention to provide a milling apparatus and a milling method capable of obtaining a powder having a desired grain size distribution, by maintaining the amount of the object material in the mill at a constant level, and to provide a compact and a sintered body manufactured from the powder obtained by this milling method.
According to an aspect of the present invention, there is provided a milling apparatus comprising; a mill for milling of an object material, a raw material feeder for supply of the object material to the mill, a collecting tank for collection of a powder obtained by the mill, a weight detector for detection of a weight of the mill holding the object material remaining therein, and a controlling portion for control of an amount of supply of the object material to the mill based on an output from the weight detector, for maintenance of a constant amount of the object material present in the mill.
According to another aspect of the present invention, there is provided a milling method using a mill milling a supplied object material and discharging an obtained powder out of the mill, comprising; a first step of detecting a weight of the mill holding the object material remaining therein, and a second step of controlling an amount of supply of the object material to the mill based on a result of the detection in the first step, for maintenance of a constant amount of the object material present in the mill.
According to the present invention, the weight of the object material remaining in the mill is detected by detecting the weight of the mill holding the remaining object material. Thus, change in the weight of the object material remaining in the mill can be detected without time delay. Therefore, according to the present invention, as compared to a method, for example, of measuring the weight of the collecting tank which collects the powder discharged from the mill, the amount of feed of the object material into the mill can be varied more appropriately without time delay. As a result, it becomes possible to eliminate hunting caused by over-feeding, and maintain the amount of the object material in the mill at a constant level. Further, by detecting the weight, the weight deviation from a target value can be detected. Therefore, the amount of supply can be varied in accordance with the deviation, making possible to bring the amount of the object material in the mill close to the desired amount more quickly. As a result, the powder having a uniform grain size distribution can be obtained.
It should be noted here that in the present specification the term the xe2x80x9cobject material remaining in the millxe2x80x9d means the object material sedimented in the mill.
According to the present invention, preferably, the controlling portion controls the amount of supply of the object material by a proportional-plus-integral-plus-derivative control. By controlling the amount of supply of the object material by the proportional-plus-integral-plus-derivative control as above, the amount of the object material in the mill can be brought closer to the desired value, making possible to reduce fluctuation. Therefore, the grain diameter of the powder can be maintained within a constant range.
Further, preferably, the weight detector includes a plurality of detecting devices. By detecting the weight of the mill using the plurality of the detecting devices, it becomes possible to accurately detect the weight of the mill even if the mill is tilted or if the weight distributes unevenly to the detecting devices. Therefore, the object material can be supplied accurately relying on the detecting devices provided in each of the supporting portions.
Further, preferably, the mill includes a mill main body, and three supporting portions provided at an equidistant interval on a circumference of the mill main body, and further, each of the supporting portions is provided with the detecting device. By supporting the mill with three equidistant supporting portions as above, the mill can be supported stably without chattering. Therefore, the weight of the mill can be detected more accurately by the detecting device provided in each of the supporting portions.
Preferably, the mill includes the mill main body and the supporting portion for support of the mill main body at a position above the center of gravity of the mill. In this case, the mill can be prevented from tilting, and the weight of the mill can be detected accurately.
Further, preferably, the mill further includes a classifying rotor provided inside and at an upper portion of the mill main body for classification of the powder. If the classifying rotor is provided inside and at the upper portion of the mill main body, the center of gravity of the mill becomes higher, and the mill becomes more apt to tilt. Even with such a classifying rotor as this, by supporting the mill main body at a position above the center of gravity of the mill, the mill can be prevented from tilting, and the weight of the mill can be detected accurately.
Further, preferably, the milling apparatus further comprises a flexible pipe provided between the raw material feeder and the mill, and another flexible pipe provided between the mill and the collecting tank. By interposing the flexible pipes as described above, the weight of the object material remaining in the mill can be detected accurately without being affected by an amount of the object material in the raw material feeder or change in the amount of the powder in the collecting tank.
According to the present invention, preferably, a value corresponding to a weight of the object material remaining in the mill during the milling is set, a deviation is calculated based on an output from the weight detector and said value, and the amount of supply of the object material is determined based on a current deviation and a previous deviation. By determining the amount of supply of the object material based on the current deviation and the previous deviation as described above, it becomes possible to supply the object material so as to bring the amount of the object material in the mill close to the desired amount more quickly, and prevent the amount of the object material in the mill from fluctuating wildly, making possible to obtain a uniform grain size distribution of the powder. Further, preferably, a first value corresponding to the weight of the object material remaining in the mill during the milling, and a second value corresponding to a weight greater than the weight of the object material corresponding to the first value are set, and supplying operation of the object material when the object material is supplied before a milling operation of the mill is controlled based on the output from the weight detector and the second value. In this case, the supply of the object material to the mill can be controlled based on the output from the weight detector not only during the milling operation but also during the time when the object material is supplied before the milling operation of the mill. Further, by making the amount of supply of the object material at the time before starting the milling operation of the mill greater than the weight of the object material remaining in the mill during the milling operation, it becomes possible to compensate in advance for a decrease in the amount of object material in the mill occurring immediately after starting the operation. Therefore, it becomes possible to obtain a uniform grain size distribution of the powder immediately after starting the milling operation of the mill and thereafter.
Further, by making the amount of supply of the object material at the time before starting the milling operation of the mill greater than the weight of the object material remaining in the mill during the milling operation, it becomes possible to compensate in advance for a decrease in the amount of object material in the mill occurring immediately after starting the operation. Therefore, it becomes possible to obtain a uniform grain size distribution of the powder immediately after staring the milling operation of the mill and thereafter.
Further, preferably, a third value corresponding to the weight of the object material remaining in the mill during the milling, and a fourth value corresponding to a weight smaller than the weight of the object material corresponding to the third value are set, and the milling operation of the mill is controlled based on the output from the weight detector and the fourth value. In this case not only during the milling operation of the mill but also in stopping the milling operation of the mill, the supply of the object material to the mill can be controlled based on the output from the weight detector. Further, by determining the stopping of the milling operation of the mill based on the weight of the object material remaining in the mill, it becomes possible to stop the milling operation before the weight of the object material remaining in the mill becomes not greater than the predetermined value. Therefore, it becomes possible to obtain a uniform grain size distribution of the powder at the time when the milling operation of the mill is being stopped.
Preferably, the object material is a rare-earth alloy. Such a rare-earth alloy as above is compacted after the milling for use as a magnet for example. If the milling is performed by the method described above, a uniform compact can be obtained due to the uniform grain size distribution.
According to another aspect of the present invention, there is provided a sintered body manufactured from a powder obtained by a milling method of milling an object material inside a mill, the method comprising steps of: detecting a weight of the mill holding the object material remaining therein; and controlling an amount of supply of the object material to the mill based on a result of the weight detection, for maintenance of a constant amount of the object material present in the mill.
By manufacturing a powder using the milling method described above, the powder having a uniform grain size distribution can be obtained, leading to an improved yield in a pressing step. Therefore, if a sintered body is manufactured from such a powder as above, yield of the sintered body can be improved.
The object described above, other objects, features, aspects and advantages of the present invention will become clearer from description of an embodiment to be made hereinafter with reference to the attached drawings.